Fiber reinforced composite structures often require two or more elements to be joined along an edge of at least one of the elements to form a joint, or node. For example, a structure may have fiber reinforced woven planar elements arranged at intersecting angles such that the elements intersect along an edge of at least one of the elements. A structure may include planar non-woven elements, for example metal sheets or plates, or ceramic panels or panels of other composition, which intersect along at least one edge.
Joining of intersecting elements may be useful in the production of grid-like structures or composite panels or skins with reinforcing ribs. In such cases, it may be desirable or necessary to provide a joint between the elements along the line of intersection. One method of achieving such a joint is to form the second element to be joined to a first element longer than necessary. The excess material of the second element is folded parallel to and placed adjacent to the first element such that a portion of the second element overlaps a portion of the first element to form a single lap joint with the overlapping portions. The overlapping portions may be fixed to each other by mechanical means, such as threaded fasteners or rivets, or chemical means, such as adhesives or resins as known in the art. Many typical applications rely upon adhesives or resins to fix the elements together.
In some cases, the second element assumes a general L-shape with one leg of the L (either the vertical or horizontal leg) parallel and adjacent to the first element. In forming a grid-like structure, a series of intersecting first and second elements can be formed as described above to form the desired grid-like structure. In some grid-like structures, first elements may be arranged in parallel rows with intersecting second elements arranged as required. Between adjacent first elements, the second elements may take on a general C-shape with approximately linear and parallel top and bottom segments, and a middle segment between, and mutually perpendicular to, the top and bottom segments. A similar result may be obtained with a second element in a general Z-shape with approximately linear top and bottom segments parallel to each other and perpendicular to a generally linear middle segment. In some cases, the middle segment may not be perpendicular to the top and bottom segments. In either form, the top and bottom segments of the second element can be fixed to adjacent first elements forming a grid-like structure.
According to known methods, the node or joint between the first and second elements is formed by a single lap shear joint parallel to the first elements. The single lap shear joint provides the structural integrity of the node with the strength of the node reliant upon the means used to join the first and second elements. Loads transferred from one element to another through the node necessarily go through the joint between the elements.
A drawback associated with nodes formed by single lap shear joints is that the strength of the joint is determined by the strength of the lap joint between the overlapping portions. Because the elements remain structurally separate, stresses applied to, or transferred to, the intersections are typically carried by the adhesive alone. The strength of the joint therefore depends on the strength of the adhesive, which in most cases is less than the strength of the fiber-reinforced elements. The difference in strength may be significant. Joints relying on adhesive alone are often the weak point in the structure.
In addition to poor strength at the nodes, present methods for fabricating a grid-like structure require significant manual labor in placing individual pieces of woven fabric in a mold, and wrapping portions of the mold with the fabric to form the open cell portion of the grid.
The present invention addresses the shortcomings of the prior art by providing a preform comprising a reinforced node and a method of fabricating a composite structure with a reinforced joint or node between elements using the claimed preform. The invention also provides a method of fabricating grid-Eke structures with reinforced nodes that is both less labor intensive and costly than prior methods.
Citation or identification of any document in this application is not an admission that such a document is available as prior art to the present invention.